Noble metal-containing supported catalysts are used in many industrial fields such as, for example, the synthesis of chemical compounds, the conversion of harmful substances in the exhaust gases from internal combustion engines and as electrocatalysts for fuel cells, to mention only a few fields of application.
To produce the highest possible catalytic activity for the noble metal, they have to be applied to the surface of the particular support material in the highest possible dispersion with particle sizes in the range between 1 and 15 nm. A small particle size in itself, however, is not a guarantee of high activity. A poorly developed crystal structure in the platinum particles thus also leads to diminished catalytic activity.
Similar considerations also apply to the quality of alloy formation of alloy catalysts. It is known in the art that ternary alloy catalysts for fuel cells with an ordered crystal structure have a catalytic activity for the electrochemical reduction of oxygen which is at least twice as great as that of a non-alloyed platinum catalyst. The catalyst is prepared by depositing the alloy components on the support material by impregnation. The alloy is formed by thermal treatment at 900° C. for a period of one hour under an atmosphere of nitrogen.
Support materials which are used for supported catalysts include a variety of materials. In general, the support materials, depending on the field of application, all have a high specific surface area, the so-called BET surface area (measured by nitrogen adsorption, in accordance with DIN 66132), of more than 10 m2/g. For fuel cells, electrically conductive carbon materials are used as supports for the catalytically active components. In the case of car exhaust catalysis, however, oxidic support materials such as, for example, active aluminium oxides (for example γ-aluminium oxide), aluminium silicate, zeolite, titanium oxide, zirconium oxide, rare earth oxides or mixtures or mixed oxides thereof are used.
Precursor compounds of the catalytically active components are deposited on the surface of these materials and are converted into the final catalytically active form by subsequent thermal treatment. The fineness of distribution (dispersion) of the catalytically active particles in the final catalyst, and thus the catalytic metal surface area available for the catalytic process, depends critically on the type of process and method used for these two processes (deposition and thermal treatment).
A variety of processes has been disclosed for deposition of the catalytically active components on the powdered support material. These include, for example, impregnation with an excess of impregnation solution. In this case an aqueous solution of the catalytically active components is added to the powdered support material, when the volume of the solution may be substantially greater than the water absorption capacity of the support material. Thus a material is produced which has a thick pasty consistency and which is dewatered, for example, in an oven at elevated temperatures of 80 to 150° C. Chromatographic effects may take place during the dewatering of this material which can lead to non-uniform distribution of the catalytically active components on the support material.
For pore volume impregnation, an amount of solvent is used to dissolve the catalytically active components which corresponds to about 70 to 110% of the absorption capacity of the support material for this solvent. The solvent is generally water. This solution is distributed as uniformly as possible, for example by spraying over the support material which is being rolled about in a tank. After distribution of the entire solution over the support material the latter is still free-flowing, despite the water content. Chromatographic effects can be largely avoided using pore volume impregnation. This method usually provides better results than the impregnation process using an excess of solvent described above.
For a process for so-called homogeneous deposition from solution, the support material is first suspended in, for example, water. Then an aqueous solution of precursor compounds of the catalytically active components is added using capillary injection with constant stirring. Capillary injection is understood to be the slow addition of the solution under the surface of the suspension of support material, using a capillary. As fast and as homogeneous a distribution as possible of the precursor compounds over the entire volume of the suspension is intended to be ensured by intensive stirring and slow addition. Here, some adsorption of the precursor compounds, and thus the formation of crystallisation seeds, takes place at the surface of the support material. The extent of this adsorption depends on the combination of support material and precursor compound. With material combinations which do not ensure adequate adsorption of the precursor compounds on the support material, or when chemical fixing of the catalytically active components to the support material is desired, the precursor compounds can be precipitated on the support material by capillary injection of a base into the suspension of the support material.
To complete preparation of the catalyst material, the support material coated with the catalytically active components is subjected to a subsequent thermal treatment which converts the precursors of the catalytically active components into the catalytically active form and optionally leads to the formation of an alloy. Temperatures of more than 300° C. up to 1000° C. and treatment times of 0.5 to 3 hours are required for this. Typically, batch processes are used for this in which the catalyst material is agglomerated and the noble metal particles become coarser due to the long treatment times and the sinter effects which take place. Noble metal particles up to 50 nm or larger can develop in this way. To form an alloy, temperatures above 900° C. and treatment times of at least 0.5 hours are usually required, wherein there is a risk of excessive particle growth due to sintering.
However, it is important that the catalysts have as high a surface area as possible (i.e. high dispersion) on the support in order to ensure high catalytic activity. Catalysts with average particle sizes for the noble metals of more than 20 nm are usually not very active.
Support materials coated with catalysts using known processes for treatment cannot simultaneously comply with the conflicting requirements for well developed crystallinity or alloy structure and small average particle diameters for the noble metal particles.
In an alternative process for the thermal treatment of powdered substances the powdered substances are treated in a high-temperature flow reactor. The treatment temperature in the flow reactor may be higher than 1000° C. The time of treatment may be varied between 0.01 seconds and a few minutes. Finely dispersed noble metals can then be deposited on, for example, aluminium oxide.
It has also been suggested that a turbulent or laminar burner be used as an essential source of heat. The process is thus performed in an oxidizing atmosphere and is not suitable for preparing catalysts on support materials made of carbon (graphite, carbon black), such as those used for fuel cells. The carbon black support would be oxidized and some would be burnt away.
Based on the forgoing, there is a need in the art for methods of preparing a noble metal-containing supported catalysts which have a high crystallinity or a well-developed alloy structure. There is also a need for noble metal-containing supported catalysts that have a small particle size and high dispersion.